Percent value for "proxy_ignore_client_abort"

"proxy_ignore_client_abort off" does not always cancel the dowloading from upstream if connection to the client is lost - it continues to download and caches the file.
This can cause huge bandwidth overhead if the proxied files are huge (e.g. videos, ...)

Feature request:
If the connection to the client is canceled before a specific threshold, the connection to upstream should be closed as well and the file should be discarded.

@itpp2012
The problem is:
A client asks the proxy to download a 2 GB file - but closes the connection after downloading 20 MB.
But the proxy does not close the connection to upstream - but downloads the 2 GB (even if the connection to the client is closed).